Typescript - Strict flags

Very strict flags

Overview of flags in article by Axel Rauschmayer

--strictNullChecks

Definition:

• Objects can’t take on the values of null or undefined if not explicitly specified.
• If this flag is not set, every JS object can take on null or undefined. So the types null and undefined are widened to any.
• see also this article

e.g. leads to having a [] without contextual type to get inferred to never[]. This can be made explicit by casting to type any[], e.g. [] as any[].

Explanation:

The issue is what is the element type of [] if there is no contextual type? undefined seems reasonable since there is not a better source of information available. This works in non strict null checks mode because undefined is in the domain of all types, and gets widened to any. In stictNullChecks it does not, since undefined is not any other type except undefined; and in that sense, never is a better name for this condition. So it is not never that is the issue.

taken from here

--noImplicitAny

Definition from Axel Rauschmayer:

If TypeScript can’t infer a type, you must specify it. This mainly applies to parameters of functions and methods: With this settings, you must annotate them.

Throws an error when the type of something is implicitly widened to any, e.g. when empty array literals are widened from never[] to any[].

noImplicitAny is often used together with strictNullChecks. This discussion shows how the implicit widening of never[] to any[] is disabled with strictNullChecks already to prevent noImplicitAny from throwing an error then.